JPH11111342A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery having superior charge and discharge cycle characteristic such as high capacity, high energy and small irreversible capacity, even at quick charging and discharging by adding a calcium compound inside of a battery. SOLUTION: As a main structural material of the negative electrode active material, carbon grains adhered with calcium compound is used. As a calcium compound to be adhered to the carbon grains, anhydride such as a halide and oxide is preferable, and a halide is more preferable. Most preferably is fluoride among halides, and CaF2 and CaF3 are used. As a carbon grain adhered with calcium compound for holding, any carbon grain as long as it is capable of storing and releasing lithium may be used, a carbon grain having a surface spacing (d002) especially by X-ray diffraction of 3.354-3.369 Åand crystal size in C-axial direction(Lc) of 200 Å preferable is preferable, since high capacity can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly, to a negative electrode for a lithium secondary battery having a large discharge capacity and a high output density and excellent cycle characteristics.

[0002]

2. Description of the Related Art As a negative electrode of a lithium secondary battery, lithium metal and lithium alloy have been conventionally used.
These batteries have a short circuit between the positive and negative electrodes due to precipitation of resinous lithium (dendrites) and have a short cycle life. Therefore, lithium equivalent to three times the battery capacity is required to compensate for the deterioration, and the energy density is low. There were drawbacks. In recent years, studies to use carbon particles for the negative electrode have been active in order to solve these problems. When using this type of negative electrode, especially graphite with advanced graphitization, for example, using lithium cobalt oxide for the positive electrode, the battery voltage becomes flat, and when used in portable equipment using unit cells, the superiority in terms of capacity is obtained. is there. However, when high-rate charging is performed using this graphite, the doping voltage at the time of charging becomes close to 0 V, which is a competitive reaction with the precipitation of lithium. Therefore, for example, in the configuration disclosed in Japanese Patent Application Laid-Open No. H5-299073, the surface of the highly crystalline carbon particles forming the core is
The electrode material is a carbon composite that is coated with a film containing a metal element of Group II and further coated with carbon, whereby carbon particles having a turbostratic structure on the surface are capable of intercalating lithium. At the same time, the charge / discharge capacity and the charge / discharge rate are remarkably improved due to the large surface area of the electrode. However, the irreversible capacity of carbon of the negative electrode carbon particles increased, and as a result, the energy density was not yet sufficient.

[0003]

As described above, when carbon particles and a composite material are used as a negative electrode, there is a problem that the irreversible capacity of carbon is increased and that it is difficult to manufacture an electrode. In order to solve this problem, the present invention uses a carbon particle having a calcium compound adhered and held as a main constituent material in the negative electrode active material, thereby achieving a high capacity even during rapid charge and discharge.
It is an object of the present invention to provide a lithium secondary battery having high energy density and excellent charge / discharge cycle characteristics with small irreversible capacity.

[0004]

Means for Solving the Problems When carbon is considered as the negative electrode active material, the reaction mainly occurs in the occlusion and release (intercalation, deintercalation) of lithium into carbon particles, but the reaction governs the reaction. As one of the factors,
It was found that the state of the film formed between the electrolyte and the carbon surface was involved. For example, as represented by the case where lithium metal is used as a negative electrode active material, a dense and highly ion-conductive film has excellent battery characteristics, while a thick and low-ion conductive film has a rate characteristic and It is known that cycle characteristics are poor. In that case, it is reported that the former is a coating such as lithium carbonate or lithium oxide, and the latter is a coating such as lithium fluoride. The same is conceivable for a coating formed on the carbon surface. That is, one of the factors that hinder the rate characteristics of the carbon particles is formation of a film having low ionic conductivity such as lithium fluoride on the surface of the carbon particles. The present inventors have conducted various studies in order to solve the problem of this coating, and as a result, by attaching and holding a calcium compound on the negative electrode surface, the fluorine anion present in the electrolyte comes to the interface between the electrolyte and the carbon particles. It has been found that this can be suppressed.

The calcium compound to be adhered and retained on the carbon particles may be any compound as long as it can be combined with calcium, and examples thereof include halides, oxides, sulfates, and nitrates, but are not limited thereto. Absent. Preferred are anhydrides such as halides and oxides, and more preferred are halides. Among the halides, fluorides are most preferred, and CaF ₂ and CaF ₃ can be mentioned. As a method for holding and holding the calcium compound, the calcium compound is deposited and held on the surface by a vapor deposition method, a sputtering method, a wet reduction method, an electrochemical reduction method, a gas phase reduction gas treatment method, laser ablation, etc. Examples of the method include an electrochemical treatment method and a method in which the calcium compound is adhered and held by mechanofusion or the like, but is not limited thereto.

The amount of the calcium compound to be adhered and retained is 30 wt% or less, preferably 10 wt% or less. Further, the particle size of the calcium compound adhered and held is desirably 1 μm or less.

The carbon particles for adhering and holding the calcium compound may be any carbon particles capable of occluding and releasing lithium, and particularly have a plane spacing (d002) of 3.3 by X-ray diffraction.
54 to 3.369 °, the crystal size (L
Carbon particles having c) of 200 ° or more are preferable because a high capacity can be obtained.

The carbon particles used in the present invention preferably have an average particle size of 100 μm or less. In obtaining a predetermined shape, a pulverizer or a classifier is used to obtain a powder. For example, a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air jet mill, a sieve, and the like are used. At the time of pulverization, wet pulverization in which an organic solvent such as water or hexane coexists can be used. The classification method is not particularly limited, and a sieve, an air classifier, or the like is used as needed in both dry and wet methods.

Examples of the negative electrode material that can be used in conjunction with the present invention include lithium metals, lithium alloys and the like, and chalcogen compounds and organic compounds containing lithium such as methyllithium. In addition, it is possible to insert lithium in advance into the carbon particles used in the present invention by using lithium metal, a lithium alloy, and an organic compound containing lithium in combination.

In the case of using the carbon particles having the calcium compound of the present invention adhered thereto, a conductive agent, a binder, a filler or the like can be added as an electrode mixture. Any conductive material may be used as long as it does not adversely affect battery performance. Normally, natural graphite (scale graphite, flake graphite, earth graphite, etc.), artificial graphite, carbon black, acetylene black, Ketjen black, carbon whiskers, carbon fibers and metals (copper, nickel, aluminum, silver, gold, etc.) Conductive materials such as powders, metal fibers, and conductive ceramic materials can be included as one type or a mixture thereof. Among them, acetylene black and Ketjen black are preferably used in combination. The addition amount is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight.

When the carbon particles of the present invention having the calcium compound adhered thereto are used, at least the surface layer of the powder may be modified with a substance other than the calcium compound. For example, gold, silver, carbon, nickel, copper and other materials with good electron conductivity, lithium carbonate, boron glass,
Coating a material having good ion conductivity such as a solid electrolyte by applying techniques such as plating, sintering, mechanofusion, and vapor deposition.

As the binder, there are usually used tetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, ethylene-propylene diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluoro rubber, carboxymethyl cellulose and the like. Thermoplastic tree, a polymer having rubber elasticity, a polysaccharide, and the like can be used alone or as a mixture of two or more. Further, it is desirable that a binder having a functional group that reacts with lithium, such as a polysaccharide, has its functional group deactivated by, for example, methylation. The addition amount is preferably 1 to 50% by weight, particularly 2 to 50% by weight.
30% by weight is preferred.

As the filler, any material may be used as long as it does not adversely affect battery performance. Usually, olefin polymers such as polypropylene and polyethylene, aerosil, zeolite, glass, carbon and the like are used. The addition amount of the filler is preferably 0 to 30% by weight.

As the current collector of the electrode active material, any current collector that does not adversely affect the battery formed may be used. For example, as the current collector for the positive electrode, in addition to aluminum, titanium, stainless steel, nickel, calcined carbon, conductive polymer, conductive glass, and the like, for the purpose of improving adhesiveness, conductivity, and oxidation resistance, aluminum And the surface of copper or the like treated with carbon, nickel, titanium, silver or the like can be used. As the current collector for the negative electrode, besides copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., the adhesiveness, conductivity, and oxidation resistance are improved. For the purpose of the above, a product obtained by treating the surface of copper or the like with carbon, nickel, titanium, silver, or the like can be used. These materials can be oxidized on the surface. As these shapes, in addition to the foil shape, a film shape, a sheet shape, a net shape, a punched or expanded material, a lath body, a porous body, a foamed body, a formed body of a fiber group, and the like are used. The thickness is not particularly limited, but a thickness of 1 to 500 μm is used.

[0015] In this way, a negative electrode can be obtained in which the carbon particles having the calcium compound adhered and retained thereon are used as the main constituent material of the negative electrode active material. On the other hand, as the positive electrode active material, M
nO ₂ , MoO ₃ , V ₂ O ₅ , Li _x CoO ₂ , Li _x
Metal oxides such as NiO ₂ and Li _x Mn ₂ O ₄ , TiS
Metal chalcogenides such as ₂ , MoS ₂ and NbSe ₃ , graphite intercalation compounds such as polyacene, polyparaphenylene, polypyrrole and polyaniline, and alkali metal ions such as conductive polymers and various substances capable of absorbing and releasing anions. Can be used.

In particular, when the carbon particles carrying the calcium compound of the present invention are used as a negative electrode active material, V ₂ O ₅ , MnO ₂ , and Li _x C are used from the viewpoint of high energy density.
_{oO 2, Li x NiO 2,} 3~4 such Li _x Mn ₂ O ₄
Those having an electrode potential of V are desirable. Especially Li _x Co
Lithium-containing transition metal oxides such as O ₂ , Li _x NiO ₂ , and Li _x Mn ₂ O ₄ are preferred.

As the electrolyte, for example, an organic electrolyte, a polymer solid electrolyte, an inorganic solid electrolyte, a molten salt, or the like can be used, and among them, the organic electrolyte is preferably used. As the organic solvent of the organic electrolyte, propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, esters such as γ-butyrolactone, tetrahydrofuran, substituted tetrahydrofuran such as 2-methyltetrahydrofuran, dioxolane, Ethers such as diethyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane,
Dimethyl sulfoxide, sulfolane, methyl sulfolane, acetonitrile, methyl formate, methyl acetate, N-methylpyrrolidone, dimethylformamide and the like can be used, and these can be used alone or as a mixed solvent. In addition, as the supporting electrolyte salt, LiClO ₄ , Li
PF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ S
O ₃ , LiN (CF ₃ SO ₂ ) _{2 and the} like can be mentioned. On the other hand, as the polymer solid electrolyte, a material obtained by dissolving the above-mentioned supporting electrolyte salt in a polymer such as polyethylene oxide or a crosslinked product thereof, or polyphosphazene or a crosslinked product thereof can be used. Further, Li ₃ N, L
An inorganic solid electrolyte such as iI can also be used. That is, any non-aqueous electrolyte having lithium ion conductivity may be used.

As the separator, an insulating thin film having excellent ion permeability and mechanical strength can be used. Olefin polymers such as polypropylene and polyethylene, glass fiber, and organic solvent resistant and hydrophobic
Sheets, microporous membranes, and nonwoven fabrics made of polyvinylidene fluoride, polytetrafluoroethylene, or the like are used.
The pore size of the separator is in a range generally used for a battery, and is, for example, 0.01 to 10 μm. Also,
The same applies to the thickness, which is in the range generally used for batteries, for example, 5 to 300 μm.

The reason why the charge / discharge characteristics, particularly the rate characteristics, are improved is not necessarily clear but is considered as follows. Generally, the battery often contains various impurities that do not contribute to the charge and discharge of the battery. For example, when LiPF ₆ is used for the electrolyte, it is conceivable that HF (hydrofluoric acid) is generated when the salt itself introduces impurities or reacts with a trace amount of water contained in the battery or in the solvent. On the surface of the carbon particles during lithium occlusion, a film having high ion conductivity such as lithium carbonate is formed between the electrolytic solution and the carbon particles, but when an acid such as hydrofluoric acid is present during or after the formation of the film. , Resulting in lithium halides with low ionic conductivity. Lithium halide generated at the interface between the carbon particles and the electrolytic solution is considered to be one of the causes of preventing the absorption and release of lithium and, as a result, reducing the rate characteristics of the negative electrode. Therefore, we thought that this problem could be solved by keeping hydrofluoric acid away from the interface between the carbon particles and the electrolytic solution, and tried to attach and hold a calcium compound to the carbon particles. As a result, the halogen anion,
In particular, we expected to occlude the fluorine anion by itself or that the calcium fluorine compound would keep hydrofluoric acid away from the interface between the carbon particles and the electrolyte due to its ionic effect. Reached.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments.

Example 1 Artificial graphite (particle size: 6 μm) was immersed in an aqueous solution of calcium carbonate dissolved in hydrofluoric acid, concentrated, dried at 110 ° C.
Vacuum dried at 0 ° C. for 16 hours. According to the chemical analysis, the adhesion holding amount of the calcium compound of the obtained powder A was 8.5% by weight with respect to 10.0% by weight of the charged amount composition. When the presence state of the calcium compound was examined by fluorescent X-ray diffraction, a peak pattern derived from calcium was detected. Next, when the dispersion state of the calcium compound was observed by energy dispersive electron probe microanalysis (EPMA), the calcium compound was distributed over the entire surface of the artificial graphite, and was slightly concentrated on the end surface of the artificial particles. Further observation of the size of the calcium compound particles with a transmission electron microscope showed that
0 ° particles were almost uniformly dispersed.

Example 2 Using the powder A obtained in Example 1 as a negative electrode active material, a coin-type nonaqueous electrolyte battery shown in FIG. 1 was experimentally manufactured as follows. The negative electrode active material and the polytetrafluoroethylene powder were mixed at a weight ratio of 95: 5, and toluene was added and kneaded sufficiently. This was formed into a sheet having a thickness of 0.1 mm by a roller press. Next, this was punched out into a circle having a diameter of 16 mm,
For 15 hours to obtain a negative electrode 2. The negative electrode 2 was used by being pressed against a negative electrode can 5 provided with a negative electrode current collector 7.

The positive electrode 1 is made of LiCoO ₂ as a positive electrode active material.
And acetylene black and polytetrafluoroethylene powder were mixed at a weight ratio of 85: 10: 5, and toluene was added and kneaded sufficiently. This was formed into a 0.8 mm thick sheet by a roller press. Next, this is
mm, and was dried at 200 ° C. under reduced pressure for 15 hours to obtain a positive electrode 1. The positive electrode 1 was used by being pressed against a positive electrode can 4 provided with a positive electrode current collector 6. LiPF ₆ was added to a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1.
Was dissolved in 1 mol / l, and a microporous polypropylene membrane was used for the separator 3. A coin-type lithium battery having a diameter of 20 mm and a thickness of 1.6 mm was manufactured using the above-mentioned positive electrode, negative electrode, electrolyte and separator. A battery using the powder A is referred to as a battery (A).

(Comparative Example) Powder B, which is artificial graphite (particle size: 6 μm), was used instead of powder A as the negative electrode active material.
Otherwise, the procedure of Example 2 was followed to fabricate a battery. The obtained battery is referred to as a comparative battery (B).

A charge / discharge test was conducted using these batteries (A) and (B). The charge / discharge rate is 100 / g of carbon.
mA and 200 mA, and the upper and lower limit potentials of charge and discharge were 1.0 V and 0.01 V, respectively. Table 1 shows the results of the discharge capacity at the fifth cycle.

[0026]

[Table 1]

A comparison between the battery (A) using the powder A and the powder B and the comparative battery (B) shows that when the charging / discharging rate is 100 mA / g of carbon, there is no difference in the discharge capacity. When the charge / discharge rate is 200 mA / g of carbon, the battery of the present invention (A) using powder A has a larger discharge capacity than the comparative battery (B). Although the reason for these phenomena is not clear, when using carbon particles having a calcium compound adhered and held as the main constituent material in the negative electrode active material, the state of the interface between the electrolyte, particularly the solute and the material surface, is reduced. Probably involved. That is, in the case of powder B, which is a carbon particle that does not adhere and hold a calcium compound conventionally used,
The coating on the carbon surface generated by the insertion and extraction of lithium,
It is considered that the lithium-halide was produced by reacting with a small amount of hydrogen halide present inside the battery, and the rapid charge / discharge characteristics were lowered due to the decrease in ionic conductivity. On the other hand, in the case of the powder A, which is a carbon particle in which a calcium compound is adhered and held to a main constituent material of the negative electrode active material, a small amount of hydrogen halide present inside the battery is captured before reaching the interface between the carbon particle and the electrolyte. It is also conceivable that the ionic effect of the halide serves to protect the coating of the carbon particles from hydrogen halide.

Further, comparing the initial charge and discharge efficiencies of the battery (A) and the comparative battery (B), there was almost no difference, so that the reaction occurring at the interface between the carbon particles and the electrolyte was increased. It is considered that only a decrease in ionic conductivity could be suppressed. In the above example, the carbon particles in which calcium fluoride is adhered and held as the main constituent material in the negative electrode active material have been described. However, the same effect was confirmed for other calcium compounds. Further, the same effect was obtained when a calcium compound was added to the inside of the lithium secondary battery. The present invention is not limited to the starting materials, the production method, the positive electrode, the negative electrode, the electrolyte, the separator, the shape of the battery, and the like of the active material described in the above-described embodiment.

[0029]

Since the present invention is configured as described above, the decrease in ionic conductivity at the negative electrode active material interface is small,
As a result, rapid charge / discharge characteristics are improved, and cycle characteristics are also improved. In addition, since the treatment is simple and inexpensive, it is an excellent method of reforming the negative electrode material, and the resulting battery has high capacity, high energy density, and low irreversible capacity even in rapid charge and discharge. FIG.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a coin-type non-aqueous electrolyte battery according to an example of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 positive electrode 2 negative electrode 3 separator 4 positive electrode can 5 negative electrode can 6 positive electrode current collector 7 negative electrode current collector

Claims (4)

[Claims] 1. A lithium secondary battery characterized by adding a calcium compound to the inside of the battery. 2. A lithium secondary battery characterized by using carbon particles having a calcium compound adhered and held as a main constituent material of a negative electrode active material. 3. The lithium secondary battery according to claim 1, wherein the calcium compound is a compound with fluorine. 4. The carbon particles as the negative electrode active material have a plane distance (d002) of 3.354 to 3.3 according to an X-ray diffraction method.
69 °, the crystal size (Lc) in the C-axis direction is 200 °
A lithium secondary battery as described above.